1. Field of the Invention
The present invention relates to a presensitized plate for use in making a lithographic printing plate which makes it possible to perform printing without using water to dampen the plate.
2. Prior Art
There have been proposed various kinds of presensitized plates for use in making lithographic printing plates not requiring dampening with water (hereunder referred to as "dry PS plates") in order to carry out lithographic printing operations. Among them, those plates comprising a substrate coated thereon with a photopolymerizable light-sensitive resin layer and a silicone rubber layer in that order show quite excellent properties, as disclosed in, for instance, Japanese Patent Publication for Opposition Purposes (hereinafter referred to as "J.P. KOKOKU") Nos. Sho 54-26923 (U.S. Pat. No. 3,894,873) and Sho 55-22781.
In those dry printing plates, the silicone rubber layer is generally obtained by partially crosslinking macromolecular polymers having a polysiloxane chain as a main skeleton with a crosslinking agent. The following two methods for curing the silicone rubber layer are known:
(1) a condensation method wherein an organopolysiloxane having hydroxyl groups at both terminal ends is crosslinked with a silane or siloxane having a hydrolyzable functional group to obtain silicone rubber; and
(2) an addition method wherein an addition reaction of a polysiloxane containing .ident. Si--H groups and a polysiloxane containing --CH.dbd.CH-- groups is carried out to obtain a silicone rubber as disclosed in Japanese Laid-open Patent Application (hereinafter referred as "J.P. KOKAI") No. 61-73156 and Japanese Patent Applications No. 1-301568.
The condensate type silicone rubber produced by method (1) has the drawback that it is difficult to obtain a stable dry PS plate which does not fluctuate in sensitivity. This is because curability and adhesion to the light-sensitive layer may vary depending on the moisture content during curing. Therefore, the adduct type silicone rubber which does not have the above drawback would be superior.
As the photosensitive resin layer of the positive-working presensitized plate for use in making a lithographic printing plate (hereinunder referred to as a "PS plate"), generally utilized is a photopolymerizable type light-sensitive composition which is curable by light exposure. In the image production process of the PS plate having the foregoing layer structure, the non-image areas are produced by curing the light-sensitive composition by light exposure and, if necessary, by causing photoadhesion at the interface between the silicone rubber layer and the light-sensitive layer to ensure firm adhesion between the two layers and to avoid the penetration of the developer into the photosensitive layer and hence the dissolution of the light-sensitive composition. 0n the other hand, the image areas are produced by penetrating the developer into the light-sensitive layer through the silicone rubber layer to partially or fully dissolve the light-sensitive resin layer and physically removing the silicone rubber layer. In such an imaging process, it has been difficult to simultaneously sufficiently avoid background contamination (the term "contamination" refers small spots of printing ink which deleteriously adhere to the non-imaged areas), printing durability and tone reproducibility with a dry PS plate containing a conventional adduct type silicone rubber layer. That is, the printing durability can be improved by raising the degree of curing, but the higher the degree of curing the less well background contamination is avoided and the poorer the tone reproducibility. On the other hand, a lower degree of curing may make it possible to avoid the contamination better (referred to as "anti-contamination characteristics" hereinafter) and improve tone reproducibility, but it adversely affects the printing durability.